The technology of controlling sulfur trioxide in combustion effluents using magnesium oxide is well established but cannot always be employed without causing problems that may offset a projected advantage.
Sulfur trioxide (SO3) is a byproduct of burning fuels that contain sulfur, which is oxidized during combustion. It primarily forms sulfur dioxide (SO2). A portion of the sulfur dioxide is quickly oxidized to SO3 by homogeneous and heterogeneous reactions. As the combustion gases work their way through the combustor, associated equipment and ductwork, more of the SO3 is formed and can cause corrosion and plume.
SO3 vapor readily converts to gaseous sulfuric acid when combined with water vapor in the combustion gases. As gas and surface temperatures cool through the combustor system, the SO3 vapors form a fine aerosol mist of sulfuric acid. The acid aerosol contains sub-micron particles of acid, which can evade separation or capture in gas cleaning devices and exit the stack. Even relatively low SO3 concentrations exiting the stack cause significant light scattering and can easily create a visible plume giving a high-opacity reading. As a general rule, every 1 part per million by volume of SO3 will contribute from 1 to 3% opacity. Thus, exhaust gas concentrations of only 10 to 20 ppm SO3 can cause opacity and acid plume problems. In addition, deposition or formation of acid on any metal surfaces below the acid dew point causes corrosion within the unit, commonly referred as cold-end corrosion that affects all equipment along the flue gas path such as the air heater, duct work and stack liners.
Burning heavy liquid fuels that contain vanadium as well as sulfur can make plume and acid corrosion particularly difficult to control. Slagging, plume and corrosion problems can be particularly acute for fuel oils produced from Venezuelan, Saudi Arabian, and Mexican crude and Canadian tar sands. These fuels, as well as No. 6 oil and others, will result in SO2 generation and can cause many problems for boiler operators—including high-temperature slagging and fouling and related eutectic corrosion, cold-end corrosion and fouling, and opacity issues related to particulates and acid mist. In the combustion zone, sulfur in the oil (e.g., 1-5%) forms SO2, some of which is oxidized to SO3, which can condense as sulfuric acid on the back end surfaces (where the temperature has typically been reduced to less than about 150° C.) and promote corrosion and acid plume. SO3 can result from oxidation by SCR catalysts, which often contain vanadium, and other metals, as well as metals in the fuel. Vanadium oxides, in particular, have been found to accumulate in deposits on heat exchange and duct surfaces and cause oxidation of SO2 to SO3.
Vanadium is a metallic element that chemically combines with sodium and oxygen to produce very aggressive low melting point compounds implicated in accelerated deposit formation and high-temperature corrosion. Vanadium is oil-soluble, and vanadium content varies in fossil fuels such as crude oil, coal, oil shale, and tar sands. The vanadium problem can be particularly acute for fuel oils produced from Venezuelan, Saudi Arabian, and Mexican crude and Canadian tar sands. In crude oil, concentrations up to 1200 ppm have been reported. When such oil products are burned, the traces of vanadium may initiate corrosion in boilers and will attach to heat exchange surfaces and catalyze the oxidation of SO2 to SO3. Vanadium cannot presently be economically reduced or removed by the refinery, so the use of combustion and post-combustion treatment methods and chemical additives are essential for fuels high in vanadium. While improvements have been made, there is yet to be found a methodology that can meet varying fuel sources and varying combustor loads with desired combustor heat outputs and reliability.
For SO3 control, injection of alkali material such as magnesium oxide (typically introduced as magnesium hydroxide) can be useful; but it can result in accumulation of solids along the furnace floor and duct walls. Solids accumulation may lead to an outage of a combustor or a process as well as inefficient reagent use and added expense of solids disposal. Another adverse effect of introducing magnesium oxide is that it tends to lighten the color of the heat exchange surfaces and thus making them reflective, causing reduction in their heat exchange efficiency. The combined effect on heat exchange efficiency and solids accumulation cannot be tolerated in some combustors.
Not all alkaline treatment agents will be useful. For example, lime cannot be practically used to eliminate the SO3 because it reacts with SO2 to form gypsum, which can create severe fouling problems. Gypsum forms a hard, non-friable deposit with very low solubility that is difficult to remove.
In systems that may include a NOx reduction operation, such as SNCR or SCR processes, SO3 causes a problem by reacting with water vapor and ammonia present due to NOx reduction chemicals. The result can be reaction to form ammonium sulfate and ammonium bisulfate. Both of these ammonia salts can cause fouling and corrosion problems in the system. Ammonium bisulfate has a melting point under 300° F. and ammonium sulfate at just over 450° F., making both molten or tacky at typical air heater operating temperatures and making it possible for them to coat, foul and corrode the air heater. However, the introduction of magnesium oxide in advance of an SCR catalyst may not sufficiently address SO3 created within the catalyst. And, waiting for SO3 remediation until after the SCR unit can leave hot-end slagging and corrosion problems untreated and result in such high SO3 concentrations at the cold end that residence times for treatment and chemical dosages will be far too costly. For many boilers, operators have been forced to let the problems occur and then shut the combustor down for cleaning—a clearly undesirable option.
There is a present need for a process to solve the slagging, corrosion and plume problems for burning sulfur-containing fuels, especially those having significant vanadium contents. And, there is a need to solve as much of these problems as practical without adversely affecting the efficiency of the combustor burning the problem fuels.